1. Field of Invention
The present invention relates to a flashlight lamp circuit for cameras and, more particularly, a flashlight lamp circuit with automatic light adjustment functionality.
2. Related Art
The flashlight lamp is an essential component part of a conventional camera. Obtaining a correct light exposure usually requires knowing the distance of the object and the light sensitivity of the negative film and then evaluating the needed adequate diaphragm. The rear side of the flashlight lamp usually has a fast lookup table that provides the user with a selection of diaphragms. However, this conventional system is not very convenient, which motivates the development of flashlight lamp with automatic light adjustment.
A flashlight lamp with automatic light adjustment is usually provided with a control circuit and light sensor elements. As shown in FIG. 6, once the power source is turned on, a transistor V1 is used as switch to create intermittent pulses so that transformer T1 initially obtains an alternating voltage. Voltage increase is obtained via transformer T1, and it secondarily obtains an alternating voltage of about 60V. The alternating voltage is half commutated via diode D1 to become a direct voltage, which then charges main capacitor C2 and trigger capacitor C3. Once the flashlight lamp has been sufficiently charged, the synchronous light contact of the camera connects to the flashlight circuit. A conventional flashlight lamp circuit uses a transistor as switch controller of the charging current, and an RC circuit is used for the oscillation frequency loop. However, due to a relative high transistor-resistor-capacitor error value, the circuit stability is altered. In addition, the conventional flashlight adjustment circuit uses a Zener diode with an avalanche voltage of 270V to determine when charging stops. At this time, it is necessary to control transistor 2SC1623 to a conducting status, which then generates a high signal informing the CPU that there is voltage saturation. The CPU then outputs another signal to stop charging. Effective charging stop then is performed via transistor DTC143EK. The use of transistors as control means in association with a feedback loop therefore creates a relatively high error value, which may induce erroneous actions. Furthermore, the VR variation range is excessively large, which renders difficult initial settings of the automatic light adjustment.